Cruise control is now usual in motor vehicles, e.g. cars, trucks and buses. An object of cruise control is to achieve a uniform predetermined speed. This is done either by adjusting the engine torque to avoid retardation, or by applying braking action on downhill runs where the vehicle is accelerated by its own weight. A more general object of cruise control is to provide convenient driving and better comfort for the vehicle's driver.
A driver of a vehicle equipped with cruise control usually chooses a set speed vset as the speed he/she wishes the vehicle to maintain on level roads. A cruise control then supplies an engine system of the vehicle with a reference speed vref used for control of the engine. The set speed vset may thus be regarded as an input signal to the cruise control, whereas the reference speed vref may be regarded as an output signal from the cruise control and is used for control of the engine.
Today's traditional cruise control (CC) maintains a constant reference speed vref usually set by the vehicle's driver in the form of a set speed vset which is thus here a desired speed chosen for example by him/her, and for today's conventional cruise controls the reference speed is constant and equal to the set speed, i.e. vref=vset. The value of the reference speed vref changes only when adjusted by the driver while the vehicle is in motion. The reference speed vref is then sent to a control system which controls the vehicle so that its speed corresponds when possible to the reference speed vref. If the vehicle is equipped with an automatic gearchange system, the gears may be changed by that system on the basis of the reference speed vref to enable the vehicle to maintain the reference speed vref, i.e. to enable it to maintain the desired set speed vset.
In hilly terrain, the cruise control system will try to maintain the set speed vset uphill and downhill. This may result inter alia in the vehicle accelerating over the crest of a hill and into a subsequent downgrade. It will then need to be braked to avoid exceeding the set speed vset or will reach a speed vkfb at which the constant speed brake is activated, which is a fuel-expensive way of driving the vehicle. It may also need to be braked downhill to avoid exceeding the set speed vset or the constant speed brake's activation speed vkfb in cases where the vehicle does not accelerate over the crest of the hill.
To reduce fuel consumption, especially on hilly roads, economical cruise controls such as Scania's Ecocruise® have been developed. This cruise control tries to estimate the vehicle's current running resistance and also has information about its historical running resistance. The economical cruise control may also be provided with map data comprising topographical information. The vehicle is then located on the map, e.g. by means of GPS, and the running resistance along the road ahead is estimated. The vehicle's reference speed vref can thus be optimised for different types of roads in order to save fuel, in which case the reference speed vref may differ from the set speed vset. This specification refers to cruise controls which allow the reference speed vref to differ from the set speed vset chosen by the driver, i.e. reference speed-regulating cruise controls.
An example of a further development of an economical cruise control is a “look ahead” cruise control (LACC), a strategic form of cruise control which uses knowledge of sections of road ahead, i.e. knowledge of the nature of the road ahead, to determine the reference speed vref. LACC is thus an example of a reference speed-regulating cruise control whereby the reference speed vref is allowed, within a certain range [vmin, vmax], to differ from the set speed vset chosen by the driver, in order to achieve more fuel saving.
Knowledge of the road section ahead may for example comprise information about prevailing topology, road curvature, traffic situation, roadworks, traffic density and state of road. It may further comprise a speed limit on the section ahead, and a traffic sign beside the road. Such knowledge is for example available from location information, e.g. GPS (global positioning system) information, map information and/or topographical map information, weather reports, information communicated between vehicles and information provided by radio. All this knowledge may be used in a variety of ways. For example, information about a speed limit on the road ahead may be used to achieve fuel efficiency by lowering the vehicle's speed before reaching a lower speed limit. Similarly, knowledge of a road sign which indicates for example a roundabout or intersection ahead may also be used to achieve fuel efficiency by braking before the vehicle reaches the roundabout or intersection.
An LACC cruise control does for example make it possible, before a steep upgrade, for the reference speed vref to be raised to a level above the set speed vset, since the vehicle will be expected to lose speed on such a climb owing to high train weight relative to engine performance. Similarly, before a steep downgrade, the LACC cruise control makes it possible for the reference speed vref to be lowered to a level below the set speed vset, since the vehicle will be expected to accelerate on such a downgrade owing to its high train weight. The concept here is that reducing the speed at which the vehicle begins the downhill run makes it possible to reduce the energy braked away and/or the air resistance losses (as reflected in the amount of fuel injected before the downgrade). The LACC cruise control may thus reduce fuel consumption without substantially affecting journey time.
In hilly terrain, a reference speed-regulating cruise control, unlike a conventional cruise control, can thus actively vary the vehicle's speed. For example, the vehicle's speed will be reduced before a steep downgrade to enable it to utilise more of the cost-free energy provided by the downgrade, instead of braking it away. The speed may also be increased before a steep climb to prevent the vehicle from losing too much speed and time.
A problem with reference speed-regulating cruise controls is that it is difficult for the control system to determine how much the cruise control is to be allowed to vary the reference speed vref.
This is because external parameters, e.g. traffic situation, driver temperament and terrain may also affect the range within which it is appropriate to allow the reference speed vref to vary in a specific situation. A wider speed range generally results in more fuel saving but also in large speed variations which may be disturbing to surrounding traffic.
In the aforesaid Scania Ecocruise® function there is a tightly specified speed range between the truck's speed limit (which is often 89 km/h) and a lower speed which is 20 km/h below the set speed but never below 60 km/h.
US-2003/0221886 refers to a cruise control in which speed ranges are set. The system can anticipate downgrades and upgrades and incorporate them in the calculations, but the specification gives no details about how this is done in practice.
DE-10 2005 045 891 refers to a cruise control system for a vehicle which sets a range within which the vehicle's speed is allowed to vary. The object is inter alia to account for the wind conditions to which the vehicle is subject.
JP-2007276542 refers to a cruise control in which the vehicle's speed is allowed to fluctuate relative to a predetermined speed in order to reduce fuel consumption.